The present invention relates to synchronization methods and apparatus for a base station and a user equipment as wireless communication devices, and more particularly, to apparatus and methods for enabling fast synchronization between base stations while the base station dynamically sleep and wake up, as well as apparatus and methods for dynamically and flexibly providing network synchronization for the user equipment utilizing different types of base stations.
Explosive growth of the mobile Internet traffic, as well as the “Internet of Things” trend led by wearable devices, has brought challenges to cellular network in terms of efficiency and flexibility. On one hand, to support the fast growing requirements for traffic amount and traffic rate, the deployment density of base stations which provide wireless access in cellular network is increasing higher. Since the base station part occupies most of the energy consumption in the entire cellular network, the intensification of the base stations causes the energy consumed by the entire network significantly increase. A large part of the base station energy consumption comes from basic power consumption, i.e. the power consumption when there is no radio frequency data transmission, thus sleep of the base station is considered to be one of the approaches for effectively improving network energy efficiency. However, in traditional cellular network, the base stations of various cells are under distributed deployment. Each base station makes decisions independently and lacks cooperation with each other, such that there are holes in network coverage left by a base station after entering sleep. Users in the holes are unable to receive the network service. On the other hand, the popularity of new types of intelligent devices (e.g., wearable devices) advances the demand for inter-device communication. Different from the high capacity requirements for traditional cellular network traffic (e.g., video streaming media traffic), inter-device communication requirements exhibit features such as short packets, small flow, high reliability guarantee. Intensively deployed small cells alone are unable to flexibly provide different services for different traffic requirements well.
In order to meet the challenges on efficiency and flexibility, patent reference 1 (CN104080178A) presents a separation architecture called super cellular. It separates the network coverage into two layers (control coverage and traffic coverage) at the over-the-air interface, accordingly dividing cellular network base stations into two categories: control base station and traffic base station. A control base station remains open to provide access guarantee for cellular network users. Whereas the traffic base station deploys flexibly according to the time-space dynamic characteristics, and dynamically opens and closes (sleep and wake up of the base station), thereby providing network service with high energy efficiency.
However, this separation architecture brings new challenges on the synchronization of cellular network. First, under separation architecture, the base stations have two categories: Control Base Station (CBS) and Traffic Base Station (TBS). User Equipment (UE) is also more and more versatile (e.g., intelligent cellphones, tablet computers, wearable devices, etc.). The more types of network devices increase the complexity and cost of synchronization. Second, under the centralized control of the control base station, the traffic base station will be dynamically assigned to provide high speed data service for users. Base stations without service requirements may be switched to a sleep state. These dynamic base station operations make operation of the network become more flexible, and meanwhile require the network synchronization more flexible and reliable. Third, while the traffic base station is in sleep, when a new high rate traffic request arrives, the control base station may wake up the traffic base station in sleep to provide service for the new traffic request. Whereas the just waked up traffic base station may have been out of synchronization with the network. This requires providing a fast and efficient synchronization scheme for the waked up traffic base station. Moreover, to enable the user traffic to seamlessly switch between multiple traffic base stations, and to utilize multiple traffic base stations to cooperatively provide data service for a single user, the synchronization precision required by the network would be on the order of resource block, and in modern cellular systems would be on the order of sub-microsecond. However, patent reference 1 is focus on network architecture design as well as signaling payload reduction methods, and does not provide the methods for cellular network synchronization under separation architecture.
In patent reference 2 (U.S. Pat. No. 6,373,834B1), presents a method for cellular network synchronization. In this method, a master timing unit and a slave timing unit achieve synchronization by exchanging synchronization signaling which carries time information. Here, the master timing unit and slave timing unit may all serve as a timing unit which starts the synchronization operation. However, this synchronization method is only applicable to wired connected backhaul network, unable to provide synchronization for mobile user equipment, neither providing synchronization scheme for base station in sleep.
In standard reference 3 (IEEE 1588-2008: 24 Jul. 2008. 1.1 Scope), presents the second version of Precision Time Protocol (PTP). Here describes a protocol which provides precise clock synchronization for packet switching wired network. This protocol can achieve a synchronization precision on the order of sub-microsecond within the fast wired local area network. However, it does not involve the time synchronization under wireless network connection, neither does it involve the solution for synchronization in a scenario where the nodes dynamically sleep and wake up.
In standard document 4 (3GPP TR 36.872 V12.1.0 (2013-12). 6.3 Feasibility and benefits of radio-interface based synchronization mechanisms), discusses the feasibility and potential advantages of using radio frequency wireless interface to achieve inter-cell synchronization under existing cellular network standard LTE. Here presents a scheme that achieves synchronization between a small cell and the network as well as synchronization between small cells by way of network listening based on existing signals in LTE. To improve the synchronization precision, the reference considers the way of transmission muting, but it will degrade the network performance. Moreover, there is no frequency synchronization method provided in the reference, and it does not involve the base station synchronization method after introducing base station sleep operation.
In standard reference 5 (3GPP TS 22.042 V11.0.0 Release 11), it specifies a function called Network Identify and Time Zone (NITZ). This function enables a base station to provide time service for user equipment. However, this function is optional. Some network operators do not support this function. In addition, the time precision required by NITZ is only on the order of minute. This scheme cannot provide guarantee for the synchronization precision required by the real-time mobile Internet applications.